The three dimensional structure of the heat stable protease thermolysin has been determined from a 2.3 A resolution electron density map. We propose to improve the precision of the structure determination by a sequence of refinement steps, and by extending the resolution of the X-ray diffraction data to at least 2.0 A resolution. This project is well under way. As part of the refinement process we will continue to apply Fast Fourier Transform techniques to protein crystallography. These methods can decimate the computing time necessary for refinement of large structures, and also allow new methods of protein refinement which we propose to test with the thermolysin structure. We will continue X-ray structure analyses of biologically interesting proteins. In particular the structure analysis of T4 phage lysozyme is well advanced, and we hope to obtain a 2.5 A resolution electron density map during the coming year. Also we will continue a structural study of a bacteriochlorophyll protein, of which suitable crystals have been obtained.